What Size Solar Battery Do I Need in the UK?

Thinking about getting a home battery but not quite sure what size you need?

You’re not the only one. Battery sizing is one of the most common questions people ask us - and understandably so. With battery options ranging from 5 kWh to 30+ kWh, it’s easy to feel like you’re picking lottery numbers rather than making a smart home investment.

The truth is - battery sizing isn’t complicated once you understand a few key principles. In fact, once you’ve got your annual energy usage and a sense of your goals, sizing a battery becomes surprisingly straightforward.

PS We offer MCS-certified battery installation nationwide. Simply answer these questions, get your fixed price and arrange your free design.

How Much Battery Storage Do You Need?

Battery Size vs Physical Size

Let’s clear up the biggest misunderstanding straight away.

When someone asks “What size battery should I get?” they are not talking about:

• The weight

• The height

• The footprint

• How chunky the unit looks on the wall

They’re talking about energy storage capacity, measured in kilowatt-hours (kWh).

🎥 Prefer video? Check out our YouTube video explaining how to size a battery:

Why this matters

It’s a common assumption that a larger, heavier, more imposing battery must store more energy. But battery shells vary wildly between manufacturers - and what’s inside those shells matters far more.

Different models use different:

• Cell chemistries

• Efficiency levels

• Depth-of-discharge limits

• Thermal management systems

So two “10 kWh” batteries can behave very differently depending on how much of that energy they will actually let you use.

This brings us to two key terms:

Nominal Capacity

This is the headline number - the total amount of energy the battery could hold if you drained it from 100% to 0%. Great for marketing… but rarely what your home receives.

Usable Capacity

This is the real-world figure that actually powers your home.

Most modern batteries keep a small buffer of energy in reserve to protect the cells from damage, meaning the usable capacity is often lower than the nominal.

Examples:

• A “10 kWh battery” may only provide 8.6 kWh usable.

• A “13.5 kWh battery” (e.g., some premium systems) may offer the full 13.5 kWh usable.

In short:

When we talk about “battery size,” we mean:

👉 Usable kWh storage - not the physical size of the box.

How to Size Your Solar Battery

Below is the exact method we use in our designs.

Step 1 - Find your annual energy consumption

This is the foundation of your battery size.

You’ll find your annual kWh usage on every electricity bill. It may appear as:

• “Estimated annual consumption: 4,200 kWh”

• “Your usage last year: 5,732 kWh”

Typical UK annual usage:

• Low-usage home: ~2,500 kWh

• Average home: 3,500–5,000 kWh

• High-usage home: 6,000+ kWh

• Heat pump homes: 8,000–12,000 kWh

• EV charging: +2,000–4,000 kWh

If you’re unsure, our calculator can estimate it for you in seconds.

Step 2 - Convert to daily consumption

Divide your annual usage by 365.

Example:

5,000 kWh ÷ 365 ≈ 13.7 kWh/day

So on an average day, this home uses around 13.7 kWh of electricity.

Many guides stop here — and this is why people end up with undersized systems.

We take it a step further.

Step 3 - Apply the 60/40 Winter Split (The Heatable Method)

UK energy use changes dramatically between seasons.

In winter:

• It’s colder

• Heating systems, heat pumps and appliances work harder

• Lights are on longer

• People stay home more

• Solar production drops

• Peak-time prices increase

Your battery has to work much harder from October to March.

That’s why we use the 60/40 seasonal split:

• 60% of your annual electricity is used in winter (Oct–Mar)

• 40% in summer (Apr–Sep)

Apply it to the example:

Annual usage: 5,000 kWh

→ Winter usage = 5,000 × 0.60 = 3,000 kWh

Winter days: 182.5

→ Winter daily usage = 3,000 ÷ 182.5 ≈ 16.4 kWh/day

What this means

Your battery should ideally be sized to cover your winter daily usage.

Why?

Because in winter, solar production is low and your battery becomes your main tool for avoiding peak rates and evening grid use.

A battery sized to summer usage will feel fantastic in July…

…then disappoint every evening from October onward.

This is why most “underperforming” systems are actually just undersized.

Quick Estimation Table:

Here’s a simple shortcut to get you in the right ballpark:

This works well for quick estimates.

For a more accurate calculation, you need to know the daily average for three things: 

1. The amount of energy in KWh, your solar system is estimated to produce

2. The amount of energy generated you’re expected to export to the grid KWh

3. How much energy you use per day in KWh

All of this information will be provided to you with your MCS proposal. If you don’t have that to hand, you can use our online solar tool here to see how much different batteries can potentially save you.

Once you have that, use this formula:

This will get you a big enough battery to size your system, plus some redundancy to charge it from the grid in the winter months when solar generation is lower.

Battery Specifications: What Else Matters?

Choosing the right battery isn’t only about capacity. Two other factors are crucial.

1. Usable Capacity (kWh)

As mentioned earlier, always look at usable, not nominal.

Examples include:

Tesla Powerwall 3

• 13.5 kWh usable

• One of the highest usable/nominal ratios in the market

SigenStor Modules

• 9.1 kWh nominal

• ~8.6 kWh usable

• Two modules = 17.2 kWh usable

This is why SigenStor modules are incredibly flexible - you can get very close to your target capacity without huge jumps.

2. Charge/Discharge Rate (kW)

This is one of the most overlooked specifications.

Think of your battery like a bathtub:

• kWh = the amount of water it can hold

• kW = how fast the tap fills or drains the tub

A high-capacity battery with a weak charge/discharge rate won’t be able to:

• charge fast enough during cheap-rate windows

• discharge fast enough to keep up with your household demand

• handle multiple appliances at once

Heatable rule of thumb:

👉 Avoid anything under 3 kW.

Most premium batteries now sit between 5 kW and 11 kW.

Recommended Solar Batteries 

Here is a list of some of our favourite solar batteries with their capacity, charge/discharge rates, warranty and estimated prices:

Table last updated: November 2025.

Why Choosing the Right Battery Size Really Matters

Battery sizing directly impacts how much you save - and how much value you get from your solar or battery-only system.

Here’s why it’s so important:

1. Too Small = Constantly Running Out of Power

A battery that empties too early:

• leaves you importing expensive electricity

• reduces your savings

• can’t support your home during peak pricing

• won’t fully optimise your solar or tariffs

2. Bigger Batteries Now Let You Make Money

This is where modern tariffs change everything.

With smart tariffs like:

• Octopus Intelligent (cheap nighttime charging ~7p)

• Octopus Flux (high export, often 20p+)

…and with systems like:

• Tesla Autobidder,

• Sigen AI Trading,

…your battery can now buy low, sell high.

For example:

If your home needs 16.4 kWh/day but you have 27 kWh of storage:

• 16.4 kWh = for your home

• 10.6 kWh = spare capacity

That spare capacity can be exported through intelligent automation at peak prices.

Meaning…

👉 oversizing can literally pay for itself.

3. Expandability Matters More Than Ever

Different brands expand in different ways:

• Tesla: fixed 13.5 kWh increments

• SigenStor: flexible, granular expansion (6 kWh or 9.1 kWh blocks)

• Alpha Smile B3: modular 5.02 kWh blocks up to 30.24 kWh

If you plan to add a heat pump, EV charger or air con in future, modular expansion makes life easier (and cheaper).

Can You Add More Batteries Later? (Yes - But With Rules)

Most systems allow expansion, but you need to consider:

• Max number of batteries allowed in a series

• Voltage matching

• Chemistry matching

• Brand ecosystem restrictions

• Solar compatibility

Example:

• A Tesla Powerwall 2 system allows up to 10 units, giving you a monster system of 135–140 kWh.

• But other brands may have:

• strict limits

• smaller maximum stacks

• or require matching generation hardware

If you’re planning to expand later, tell your designer - we’ll ensure your system can grow with you.

Explore a Battery Storage Case Study: Check out an insightful Q&A session featured in one of our detailed case studies on battery storage. Hear directly from a Heatable customer in the UK and gain valuable perspectives on their experience here.

The image above features a Tesla Powerwall 3 installation performed by our team of Tesla-certified installers at Heatable.

Next Steps For Your Battery Journey:

When planning to install battery storage for your home, there are several important factors to consider. Make sure to refer to the following guides to help you make informed decisions:

• What size battery do you need? 

• What is home battery storage? 

• How much does battery storage cost?

• Battery storage without solar panels 

• Pros and cons of battery storage

• Powervault battery costs.

• Why is electricity more expensive than gas?

• Sigenergy vs Tesla

To dive deeper into these topics, head over to our advice section, check out our YouTube channel for informative videos, or read a customer case study to see how others have benefited from their battery installation. 

Get a Quote for Battery Storage Today 

Without boasting you should get your battery storage installed with us, here's why: 

• Thousands of Happy Customers: We boast an average score of 4.9 on Trustpilot, outperforming the market leader.

• Which? Trusted Trader: Heatable is proudly recognised as a Which? Trusted Trader.

• Tesla Certified Installer: Heatable is proudly certified and approved by Tesla to install the Powerwall.

• MCS Accredited: Our accreditation by the Microgeneration Certification Scheme (MCS) ensures high-quality standards.

• Consumer Protection: As members of the HIES consumer code, we provide 2-year deposit protection.

• Flexible Payment Options: Choose from multiple payment methods, including finance options.

• Fixed Price Guarantee: Enjoy transparency with no hidden costs.

• Save Your Quote: You can save your quote and decide later.

• Bespoke Battery Quote Tool: Find out which battery is ideal for your home, here.